1. Field of the Invention
The present invention relates to a wavelength converting element and a method of manufacture thereof, and in particular, to a wavelength converting element in which the peak of a vertical transverse mode with respect to a substrate of a second harmonic is made to coincide with a fundamental wave such that an improvement in the wavelength converting efficiency and an improvement in the beam shape are achieved, and to a method of manufacturing the wavelength converting element.
2. Description of the Related Art
Wavelength converting elements have been conventionally manufactured which wavelength-convert a fundamental wave into a second harmonic by using an element provided with a region (inverted domains) at which spontaneous polarization of a ferroelectric having a non-linear optical effect, e.g., a crystal such as LiNbO3 or LiTaO3, is periodically inverted.
The inverted domains used here have the feature that the inversion of the polarization proceeds along the C axis in theory. In a case, such as the case of a Z plate, in which the C axis is perpendicular to the substrate surface, the inverted domains are formed at the interior of the substrate. Even if a waveguide is formed at the surface thereof, the joining with the TE mode is poor. Thus, it is difficult to efficiently join a semiconductor laser. In order to improve the joining with a semiconductor laser, it is preferable to use an X plate. However, in a case, such as the case of an X plate, in which the C axis is parallel to the substrate surface, the inverted domains are formed only in a vicinity of the surface in order to be formed along the C axis. Thus, there are problems in that the inverted domains are only formed at a shallow position with respect to the waveguide such that there is little overlapping of the fundamental wave and the inverted domain portion, and the waveguide converting efficiency is poor.
A waveguide wavelength converting element, which uses a substrate at which the orientation of the spontaneous polarization forms a predetermined angle with respect to one surface thereof, has been proposed in Japanese Patent Application Laid-Open (JP-A) No. 9-218431. In this wavelength converting element, by inclining the C axis from the substrate surface, the inverted domains are formed toward the interior of the substrate. In this way, the overlapping of the waveguide and the inverted domains is improved, and the wavelength converting efficiency is improved.
This wavelength converting element uses a proton exchange annealing method as the method for fabricating the waveguide. Thus, a flat waveguide, at whose surface portions having a high refractive index are formed, is formed. Therefore, problems arise in that offset arises between the peaks of the fundamental wave and the second harmonic, and the peak of the second harmonic and the inverted domains are offset.
In the inventions disclosed in JP-A Nos. 9-281536 and 11-72810, a high refractive index layer is formed at the surface of the substrate so as to strengthen the confinement of the fundamental wave. In this way, the overlapping of the fundamental wave and the second harmonic is relatively improved and an attempt is made to improve the converting efficiency and the beam shape. However, in these inventions as well, there is no change from and no substantial improvement over a flat waveguide whose refractive index distribution is such that the vicinity of the surface has a high refractive index.